1. Field of the Invention
The present invention relates to a sensing device for detecting the change in an applied magnetic field, and more particularly, to a sensing device which is particularly suitable for detecting the information about the rotation of, for example, an internal combustion engine.
2. Description of the Related Art
FIG. 7 is a schematic diagram illustrating the construction of a conventional sensing device, wherein its side view and perspective view are shown in FIG. 7A and FIG. 7B, respectively.
This sensing device includes: a rotating shaft 1; a rotary magnetic material member 2 serving as magnetic field variation inducing means having at least one protruding or recessed portion, wherein the rotary magnetic material member 2 is adapted to rotate in synchronization with the rotation of the rotating shaft 1; a magnetic field detecting element 3 disposed at a location a predetermined distance apart from the rotary magnetic material member 2; and a magnet 4 for applying a magnetic field to the magnetic field detecting element 3, wherein the magnetic field detecting element 3 includes a magnetoresistance pattern (not shown) formed on a thin film plane (magnetic field sensing plane) 3a of the magnetic field detecting element 3. The magnetic field detecting element 3 is fixed to the magnet 4 via a fixing member of a non-magnetic material (not shown).
In the above construction, the magnetic field applied to the magnetic field sensing plane 3a of the magnetic field detecting element 3 changes in response to the rotation of the rotary magnetic material member 2, and a corresponding change occurs in the resistance of the magnetoresistance pattern.
FIG. 8 is a block diagram illustrating a conventional sensing device using magnetic field detecting elements.
This sensing device includes: a Wheatstone bridge circuit 11, including magnetic field detecting elements disposed a predetermined distance apart from the rotary magnetic material member 2 so that a magnetic field is applied from a magnet 4 to the magnetic field detecting elements; a differential amplifier 12 for amplifying the output signal of the Wheatstone bridge circuit 11; a comparator 13 for comparing the output of the differential amplifier 12 with a reference value and outputting a "0" signal or a "1" signal depending on the comparison result; a waveform shaping circuit 14 for shaping the waveform of the output of the comparator 13 and supplying a "0" or "1" signal having sharply rising and falling edges to an output terminal 15.
FIG. 9 is a circuit diagram illustrating a specific example of the circuit shown in FIG. 8.
The Wheatstone bridge circuit 11 includes magnetic field detecting elements 10A, 10B, 10C, and 10D disposed on the respective branches of the bridge, wherein one end of the magnetic field detecting element 10A and one end of the magnetic field detecting element 10C are connected in common to a power supply terminal V.sub.cc via a node 16, one end of the magnetic field detecting element 10B and one end of the magnetic field detecting element 10D are connected in common to ground via a node 17, the other end of the magnetic field detecting element 10A and the other end of the magnetic field detecting element 10B are connected to a node 18, and the other end of the magnetic field detecting element 10C and the other end of the magnetic field detecting element 10D are connected to a node 19.
The node 18 of the Wheatstone bridge circuit 11 is connected via a resistor to the inverting input of an amplifier 12a of the differential amplifier 12. The node 19 is connected via a resistor to the non-inverting input of the amplifier 12a and further connected via a resistor to a voltage divider serving as a reference power supply.
The output of the amplifier 12a is connected to the inverting input of a comparator 13. The non-inverting input of the comparator 13 is connected via a resistor to a voltage divider serving as a reference power supply wherein the non-inverting input of the comparator 13 is also connected via a resistor to the output of the comparator 13.
The output of the comparator 13 is connected to the base of a transistor 14a of a waveform shaping circuit 14. The collector of the transistor 14a is connected to an output terminal 15 and also to a power supply terminal V.sub.cc via a resistor. The emitter of the transistor 14a is grounded.
The operation will be described below with reference to FIG. 10.
If the rotary magnetic material member 2 rotates, the magnetic field changes in response to the protruding and recessed portions of the rotary magnetic material member 2 shown in FIG. 10A, and this magnetic field changing in magnitude is equally applied to both the magnetic field detecting elements 10A and 10D constituting the Wheatstone bridge circuit 11, whereas the magnetic field detecting elements 10B and 10C experience a magnetic field which is opposite in phase to that applied to the magnetic field detecting elements 10A and 10D. Thus, the magnetic field sensing planes of the respective magnetic field detecting elements experience a change in the magnetic field corresponding to the protruding and recessed portions of the rotary magnetic material member 2. As a result, the overall magnitude of the change in the magnetic field becomes, in effect, four times greater than that which can be sensed by a single magnetic field detecting element. Thus, the magnetic field detecting elements 10A and 10D have maximum and minimum resistances at locations opposite in phase to those where the magnetic field detecting elements 10B and 10C have maximum and minimum resistances. As a result, the voltages at the nodes 18 and 19 (mid-point voltages) of the Wheatstone bridge circuit also change in a similar fashion.
The difference between the mid-point voltages is amplified by the differential amplifier 12. Thus, as represented by the solid line in FIG. 10B, the differential amplifier 12 outputs a signal V.sub.DO corresponding to the passage of the protruding and recessed portions of the rotary magnetic material member 2 shown in FIG. 10A. The output signal of the differential amplifier 12 is substantially four times greater than can be obtained when a single magnetic field detecting element is used.
The output of the differential amplifier 12 is supplied to the comparator 13, and is compared with the reference value or the threshold value V.sub.TH. The comparator 13 outputs a "0" or "1" signal in accordance with the comparison result. The output signal of the comparator 13 is shaped by the waveform shaping circuit 14 so that the output signal has sharply rising and falling edges, and has a "0" or "1" level as represented by the solid line in FIG. 10C.
In the conventional sensing device described above, the magnetic field applied to the magnetic field detecting elements varies depending on the positioning accuracy of the magnetic field detecting elements relative to the location of the magnet. This causes a variation in the output of the differential amplifier, which in turn causes an error in the edge position of the output signal of the waveform shaping circuit, wherein the edges of the output signal should correspond exactly to the edges of the protruding and recessed portions of the rotary magnetic material member.
The above problem will be described in further detail below with reference to FIGS. 10 and 11.
In FIG. 11, when the magnetic field detecting element 3 is precisely located at a correct position with respect to the magnet 4, as represented by the solid line, the magnetic field emerging from the magnet 4 passes at a right angle through the magnetic field sensing plane of the magnetic field detecting element 3. In this case, therefore, no problems occur. However, if the location of the magnetic field detecting element 3 with respect to the magnet 4 is shifted from the correct position, the magnetic field emerging from the magnet 4 no longer passes at a right angle through the magnetic field sensing plane of the magnetic field detecting element 3. This means that the direction of the magnetic field vector varies depending on the accuracy of the location of the magnetic field detecting element 3, and a corresponding variation occurs in the bias magnetic field applied to the magnetic field detecting element 3. In particular when a plurality of magnetic field detecting elements are used, as in the example described above with reference to FIG. 9, a positioning error of the magnetic field detecting elements causes a variation in the direction of the magnetic field vector applied to each magnetic field detecting element, and a large variation occurs in the bias magnetic field applied to each magnetic field detecting element.
As a result of the above positioning error, as represented by the broken line in FIG. 10B, the output V.sub.DO of the differential amplifier 12 is shifted from the correct position represented by the solid line. The output of the waveform shaping circuit 14 and thus the output of the sensing device no longer correspond exactly to the protruding and recessed portions of the rotary magnetic material member 2, as represented by the broken line in FIG. 10C. In other words, the edges of the output signal of the sensing device are shifted from the correct positions.
In view of the above problems, it is an object of the present invention to provide a sensing device capable of a high-accuracy output signal whose edges correspond exactly to the protruding and recessed portions of a rotary magnetic material member regardless of a positioning error of a magnetic field detecting element with respect to the location of a magnet.